ENTER 0 FOR AGI. The IRS took a long time last year to process 2020 returns so the AGI didn't get entered in time. Especially if you mailed it. That works for most people. See,
How to correct the AGI in the Online version
If you can’t get it to efile you will have to print and mail it.
Be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.
Thanks for the suggested fix, but it did not work. The IRS has my correct AGI in its records and it is available in a Transcript for me. Using TurboTax I was unable to e-file using the correct AGI and the suggested workaround of $0 didn't work either. So, I had to file by Mail. Very frustrating since I had already paid TurboTax for the state e-file which was also rejected. The same thing happened last year, when I did not ask for the fee to be refunded, so this year I will seek a refund from TurboTax.
@fcrosby If you used the CD/download and paid $25 for a state e-file, but were unable to e-file the state --- you can talk to customer support for a refund of the state e-file fee. If you used online TurboTax, there is no extra fee to e-file. The fee is the same whether you e-file or file by mail.
No one in the user forum can resolve a billing issue. If you have a question about your TurboTax fees or billing, make sure you use the word “billing” in your request for help. Do not use the word “refund.”
I just resolved the same problem using the $0 for AGI, even though my AGI number matched the one in the IRS's transcript for 2020, but it took two tries. After I entered 0 and resubmitted it came back with a rejection of the spouse's AGI. If you are married filing jointly then you need to enter 0 for both people, not just the primary filer and that isn't obvious (at least in the CD/download version.) Also make sure that your PIN numbers match the ones that were used last year if you e-filed. The error is for either not matching their records (although the $0 fix is matching some secret record that we can't know about.)
Given the timing of your message there is a chance that you haven't actually mailed them yet.